Low-density Geochemical Mapping Research Articles

Soil low-density geochemical mapping of technology-critical elements (TCEs) and its environmental implications: The case of lithium in Portugal

Increased use of technology-critical elements (TCEs) like lithium (Li), and their socio-environmental impacts, make it an issue of national and global importance. In Portugal, new Li exploration/exploitation projects are a very likely scenario. Thus, it is essential to establish geochemical backgrounds/thresholds for Li in soil, which can have several applications. Here, Li contents were determined and mapped from a previous low-density geochemical survey that covered the entire continental area of Portugal, following UNESCO's IGCP 259 project recommendations. The sampling sites were chosen in undisturbed/uncultivated land to ensure a reliable representation of “natural” soils. A total of 152 samples (0–20 cm; <2 mm) were taken for this study. Soil Li analysis was carried out by Flame Atomic Absorption Spectrometry (FAAS) after aqua regia (AR) extraction (geoavailable Li), while a subset of 55 samples underwent further digestion with a strong acid mixture to measure total Li (FAAS). This was done to ascertain the relationship between the two Li fractions and its environmental significance. Soil Li spatial distribution was produced with GIS software. Median values of 14 mg/kg for geoavailable Li and 60 mg/kg for total Li were estimated from these datasets. The first value is comparable to the median Li (11 mg/kg) from an AR-extraction for agricultural/grazing soils in Europe (GEMAS project). Based on spatial analysis, Cambisols overlying granitoids in northern/central Portugal contain the highest AR-extractable Li (40 mg/kg). Such areas are recognized for hard-rock Li mineralizations, mainly associated with aplite-pegmatites. Principal Component Analysis identified an important Li-Al relationship, linked to Cambisols and Leptosols overlying granitoids/metamorphic rocks. The geoavailable/total Li ratios revealed that >60 % of the samples have a relatively high proportion (>45 %) of Li that can be mobilized/dispersed in the surface environment. These findings are intended to support the management of potential concerns regarding Li mining in mainland Portugal.

Experiences from 30 years of low-density geochemical mapping at the subcontinental to continental scale in Europe

Experiences from 30 years of geochemical mapping at the (sub)continental scale in Europe using a wide range of different sample media are reviewed and discussed with a focus on the most recent GEMAS (GEochemical Mapping of Agricultural Soils) project. Comparing results from the different surveys it is possible to come to conclusions as to how geochemical surveys at the continental scale could best be designed. High analytical quality and lowest possible detection limits are key requirements. In Europe good outcomes were achieved with the &lt;2 mm fraction of soil samples and aqua regia extraction. Focus should be on high quality of sampling and analyses and more determined parameters rather than on more samples. The sample density of 1 site/2500 km 2 provides a good overview of the processes governing geochemistry at the continental scale. Results should be extensively published by the project team to allow the dataset to be known and utilized by the wider scientific community. Thematic collection: This article is part of the Continental-scale geochemical mapping collection available at: https://www.lyellcollection.org/topic/collections/continental-scale-geochemical-mapping

Concentration, Spatial Distribution and Potential Ecological Risk of Heavy Metals in Stream Sediments of the Misiones Province, Argentina

One hundred and twenty-eight stream sediment samples were collected in the Misiones province of Argentina by the low-density geochemical mapping project of the China Geological Survey. The analyzed data were used to study the concentration, spatial distribution, local pollution level and potential ecological risk of eight heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) by factor analysis, geoaccumulation index (Igeo), enrichment factor (EF) and the Hakanson’s potential ecological risk index (Eir and RI) methods. Results showed that the background values (median) of those elements were 2.97 ppm, 0.13 ppm, 212.60 ppm, 322.53 ppm, 0.013 ppm, 64.42 ppm, 12.58 ppm, and 198.85 ppm, respectively. Except for Hg, the contents of other elements were higher than the abundance of continental crust. The spatial distribution of Cu and Zn in stream sediments were consistent, while that of other elements were different. The factor analysis results implied that Cd, Cr, Cu, Ni and Zn were mainly affected by geological background and inherited the characteristics of regional parent rocks. Furthermore, Pb was not only controlled by natural conditions but also related to human activities, while As and Hg represented the anthropogenic sources, and their concentrations were affected by human activities. The results of Igeo and EF told us that Cu was heavily polluted in stream sediments; Pb and Zn were largely mildly polluted; As, Cr, Cd, Ni and Hg were mostly nonpolluted. The ecological risks were ordered as As &gt; Cu &gt; Cd &gt; Pb &gt; Hg &gt; Cr &gt; Ni &gt; Zn. In general, the potential ecological risk of heavy metals in the Misiones province was low, but As and Cu also have a high ecological risk at some sampling points, which should be considered.

Comparison of geochemical patterns from different sampling density geochemical mapping in Altay, Xinjiang Province, China

With the continuous development of geochemical mapping, especially with the proposal of the “Chemical Earth” big science project, low-density geochemical mapping technology has received increasing attention due to “low cost and high efficiency”. Due to the special geochemical properties of Au, the distribution of Au in water sediments is so complicated that the differences of low-density and high-density Au geochemical mapping are not well known, which makes it difficult for researchers to accurately evaluate the Au anomalies from low-density geochemical mapping. This paper compares Au geochemical mapping data and geochemical patterns from ultra-low-density geochemical mapping (ULGM) (1 sample/1600 km2), super-low-density geochemical mapping (SLGM) (1 sample/180 km2) and high-density geochemical mapping (HGM) (sampling density 1 sample/km2) in Altay, Xinjiang province, China. We found that the standard deviation of Au increases with the mapping density. The median values of the HGM, SLGM and ULGM are 0.71 × 10−9, 0.77 × 10−9, and 0.94 × 10−9, respectively. Altay Au anomalous is mainly controlled by 3 factors and are consequently divided into 3 kinds of anomaly types: 1. regional-scale anomalies, caused by gold-rich stratum; 2. local Au anomalies, caused by gold mineralization; 3. anomalies around the catchment, caused by the supergene migration and enrichment of fine-grained Au. In general, as the mapping density decreases, the spatial correspondence between anomalies and geological factors become weaker. HGM can figure out the single gold deposit, SLGM can delineate Au deposits concentrated area; ULGM can delineate the Au metallogenic target area. So, the ultra-low density mapping is suitable for quickly obtaining the geochemical background of the Au region in the blank area, reflecting the large-scale enrichment behavior of Au in the supergene environment.

Enrichment characteristics of Cu and Co displayed by low-density geochemical mapping in Zambia

The Sino–Zambian Cooperation project provides national catchment sediment geochemical data for 58 elements including Cu and Co from 746 sediment samples at 736 sampling sites, corresponding to a sampling density of about 1 site per 1000 km2. The Cu and Co were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) under strict quality control using field duplicates, standard reference materials and analytical replicate samples. The 95% ranges (2.5%–97.5%) of Cu and Co concentrations were from 1.8 to 57.81 × 10−6 and from 0.54 to 22.83 × 10−6, respectively, reflecting locally unevenness distributions. The median values of Cu and Co are 5.77 × 10−6 and 2.98 × 10−6, respectively. The most noticeable anomalous patterns of Cu and Co are distributed between Lusaka and Solwezi (Lufilian Arc Belt) with the trend of increasing concentrations from southeast to northwest. The all spatial distribution patterns of Cu and Co in tectonic units, drainage basins and geomorphological landscapes show good relations with parent rocks, climate, Cu (Co) deposits and smelting plants in Lufilian Arc Belt. The Cu and Co concentrations show metallogenic belts in Mkushi area, between Lusaka and Solwezi areas, and in Livingstone area.

A regional-scale geochemical survey of soil O and C horizon samples in Nord-Trøndelag, Central Norway: Geology and mineral potential

A highly efficient, low-density sampling strategy was employed to study the geochemical expression of geological bodies and the mineral potential on the county scale in Central Norway. Soil O and C horizon samples (N=752) were collected in Nord-Trøndelag and parts of Sør-Trøndelag, and analysed for 53 chemical elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Pd, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr) and Pb isotopes in an aqua regia extraction. At the sample density of one site/36km2 the four metal deposits, which have been mined in the area within the last 50years were all detected as geochemical anomalies. In addition, a number of new anomalies that may warrant follow-up surveys were found. In terms of geology the Grong–Olden Culmination is marked by a distinct 206Pb/207Pb isotope anomaly. Geochemical differences distinguish the most important belts of mafic metavolcanic lithologies in the area. Though the Fosdalen iron ore deposit is only marked in the soil O horizon, the C horizon outlines the more prominent anomalies of possibly economic interest. Climatic factors like the input of marine aerosols along the coast are clearly visible in the soil O horizon. Low-density geochemical mapping of two sample materials provides important complementing information for the interpretation of the geochemical variation in Nord-Trøndelag county.

How robust are geochemical patterns? A comparison of low and high sample density geochemical mapping in Germany

Regional geochemical mapping projects have been carried out for mineral exploration in Germany since 1954. In the early days, geochemical prospecting methods were mainly used for exploration of ore and non-metal deposits in Germany. Mapping for the first geochemical atlas of Germany was conducted in the 1980s by the Federal Institute of Geosciences and Natural Resources (BGR) with a sample density of one sample per 3km2. Parts of eastern Germany (Flechtingen Ridge, Harz Mountains, Thuringian Forest, Slate Mountains, Ore Mountains, Granulite Mountains, the Lausitz region, and along the Elbe River) were mapped by the former Central Geological Institute Berlin (ZGI) during the same period at a density of 1.3 samples per km2. Stream sediment and surface water were sampled in all of these high-density geochemical mapping projects. The first low sample density geochemical survey (one site per 380km2) was carried out for the latest geochemical atlas of Germany, which presents comprehensive information about the regional distribution of hazardous inorganic and organic substances. A density of one sample per 5000km2 was also used in Germany for the FOREGS Geochemical Mapping Project of Europe. Geochemical stream sediment and surface water data obtained at these three different scales in different geochemical mapping projects have been compared in this study for the elements Ba, Cu, Cr, Pb and U in stream sediment as well as pH, EC and U in stream water. Comparison of overlapping high and low sample density surveys conducted in the same study areas, demonstrates that the geochemical patterns produced from low sample density surveys are very nearly the same as those from the high sample density surveys, and that they can be related to natural processes. The low sample density geochemical surveys in Germany provide element background values very similar to those obtained by high sample density mapping of the same region. In unmapped terrain, low-density geochemical mapping provides a cost-effective reconnaissance technique to establish element background values.

Major advances in exploration geochemistry, 1998–2007

ABSTRACT Exploration geochemistry continues to be a major contributor to mineral exploration programs at all scales. The decade has spanned a complete economic cycle in the minerals industry, during which there has been a change in exploration emphasis from the dominance of Au and diamonds to the addition of a range of base metals and the recent revival of U exploration. Increasing metal prices and the development of new geochemical techniques have allowed exploration activity to expand in previously under-explored terrains, including those characterized by thick, transported regolith cover. There has been significant evolution and refinement of geochemical dispersion models for transported regolith, especially processes associated with oxidizing sulphide mineralization. These models have prompted further developments in sampling methods, analytical techniques and approaches to anomaly detection. The routine measurement of fundamental soil and groundwater properties, such as pH, has been demonstrated to assist in detecting geochemical and mineralogical effects of buried mineralization on regolith. New lithogeochemical techniques have been developed for terrane selection and early discrimination of potentially mineralized units. The production of regolith-landform maps, including 3-D models, and more detailed techniques for differentiating the origin and nature of regolith materials now underpins the design of exploration programs in a number of terrains. Various exploration techniques, such as biogeochemistry and soil gas, have been revitalized by the application of new technologies. Ongoing improvements to detection limits and the reliability of multi-element analytical methods have permitted development and routine application of a wide range of selective geochemical extraction methods aimed at detecting subtle geochemical haloes developed in transported regolith cover. The results from a plethora of selective extraction orientation studies have, however, indicated the reliability of such methods to be uncertain, especially in arid terrains. Low-density geochemical mapping of large regions has been undertaken by government agencies for a variety of resource, environmental and health purposes, including attracting mineral exploration. Such maps typically demonstrate the dominance of underlying geology on regional geochemical patterns in regolith, even in industrialized regions. Philosophical and numerical approaches to defining and detecting geochemical anomalies continue to be reviewed. There is a need to supplement classical parametric outlier-detection approaches, routinely applied to geochemical data, with statistical techniques that reflect a recognition that subtle, multivariate patterns related to the geochemical effects of mineralization may emanate from within the main data cloud and not outlying or distinct clusters.

Low-density geochemical mapping and the robustness of geochemical patterns

Geochemical mapping of entire continents and, ultimately, the world is still a challenge for applied geochemists. At sample densities traditionally used for geochemical exploration (1 site per 1–25 km 2 ), geochemical mapping of whole continents is logistically extremely demanding and tremendously expensive. The first low-density geochemical surveys (1 site per 200 km 2 ) were carried out during the late 1960s in Africa. Later surveys conducted in various parts of the world had sample densities ranging from 1 site per 300 km 2 to 1 site per 18 000 km 2 . Although these surveys were deemed successful by the investigators in defining variations in background element content on a regional scale, the scientific community was sceptical that low-density geochemical mapping was possible and would provide useful information. The main area of criticism centred around the concern that at such low sample densities the resulting maps would not be robust, i.e. if the same area were resampled and remapped, different geochemical patterns would emerge. Different examples from the USA, Europe, China and Africa demonstrate that low-density geochemical mapping will result in stable and robust geochemical patterns at the continental scale. Such maps are urgently needed for a wide variety of applications.

Environmental geochemical maps of Italy from the FOREGS database

The Geochemical Atlas of Italy addresses the need for geochemical mapping of the country, based on FOREGS procedures. Data from samples of Italian topsoil, subsoil, stream water, stream sediment and floodplain sediment have been extracted from the FOREGS database and supplemented with data from eight new sample sites. In total 360 geochemical maps have been produced and spatial correlations have been found between the underlying geology and element abundances in the sampled media. The aim of the Geochemical Atlas of Italy is to document background/baseline chemical element variation at the national scale. It is found to be a suitable tool to support government decision-makers to assess trigger and action limits at the local scale, when considered in the light of the complex spatial variability of Italian geology. Maps presented here demonstrate that low density geochemical mapping is a viable tool to obtain an impression of natural element variation at the country scale and to identify areas where more detailed sampling is advisable.

LOW-DENSITY REGIONAL GEOCHEMICAL MAPPING OF GOLD AND PALLADIUM HIGHLIGHTING THE EXPLORATION POTENTIAL OF NORTHERNMOST EUROPE

The <2-mm fraction of 605 samples of the C-horizon of podzols collected from an area of 188,000 km2 in the European Arctic was analyzed for Au and Pd by a method using reductive coprecipitation to enrich Au and platinum group elements (PGE) prior to analysis. Due to the low detection limits (0.1 ppb Au, 0.2 ppb Pd) only 2 percent of all samples returned values below the detection limit for Au, and 10 percent were below the detection limit for Pd. The resulting geochemical maps show clear regional distribution patterns, notwithstanding the very low sample density of 1 site per 300 km2. All known Au and PGE occurrences are associated with geochemical anomalies and several new targets for exploration were identified. Low-density geochemical mapping of Au and Pd with low detection limits in C-horizon soils can thus be used as a relatively inexpensive tool to define prospective areas within large regions.

Sub-continental-scale geochemical mapping: sampling, quality control and data analysis issues

During 1996/97, c. 1500 samples of agricultural soils from ten northern European countries (western Belarus, Estonia, Finland, northern Germany, Latvia, Lithuania, Norway, Poland, northwestern Russia and Sweden) were collected from the A p and B/C-horizons at 750 sites. The sample sites were evenly spread over a 1 800 000 km 2 area, giving an average sample density of one site per 2500 km 2 . The &lt;2 mm fractions (Poland: &lt;1 mm) of all samples were analysed for up to 62 chemical elements following ammonium acetate, aqua regia and HF extractions, and for total element concentrations by X-ray fluorescence spectrometry. Electrical conductivity, pH (water extraction) and loss on ignition (1030°C) were also determined. Each method was applied to all the samples in one laboratory only. The analytical results were evaluated and mapped using exploratory data analysis techniques. Even at this low sample density, regional-scale geochemical patterns emerge for all elements. These patterns show the influence of factors such as geology, agriculture, pollution, topography, marine aerosols and climate, and their relative importance for the observed element concentrations in the soils. Low-density geochemical mapping of agricultural soils is a viable tool to study the geochemical processes that determine the element distribution in soils at a sub-continental scale.

Low-density geochemical mapping in Portugal

This paper reports the first results of a low-density geochemical survey covering the whole continental area of Portugal, taking both topsoils and active stream sediments as the sampling media. The data were obtained in a consistent way from 653 sites at a sampling density of 1 site/135 km2. The samples were analysed for 31 elements by ICP-AES and after quality control the following 19 were selected: Al, As, Ba, Ca, Co, Cr, Cu, Fe, K, La, Mg, Mn, Ni, P, Pb, Sr, Th, V and Zn. The baseline levels for these elements are presented. The first geochemical maps of Portugal were prepared and the geochemical patterns are generally well correlated to geological factors sometimes combined with anthropogenic influences. Some general trends can be observed between geochemical patterns and soils distribution.

Low-density geochemical mapping in Hungary

Guidelines for a low-density geochemical survey were described in 1990 by the Western European Geological Surveys. A low-density geochemical survey of Hungary was carried out in 1991–1995. The results are useful for future surveys and for the IGCP 360 project ‘Global Geochemical Baseline’. In regions with well-developed drainage systems in Hungary, 196 catchment basins of approx. 400 km 2 were delineated and flood-plain deposits sampled at their outlets. The samples were taken from 0 to 10 cm and from 50 to 60 cm depths. Samples were analysed by ICP-AES and AAS techniques in two laboratories. A Geochemical Atlas of Hungary is in preparation that will show the distribution of 25 elements in the two sampled layers. Maps for the lower layer represent regional geochemical baseline values and a geochemical subdivision of the country (maps showing the distribution of element associations) was made on the basis of factor variables. Maps constructed from the data of the upper sampling level show us the present state of contamination of the surface. The results of this survey have contributed to the establishment of guidance values for soils prepared by the Hungarian Ministry of the Environment in 1995. Safe levels were established for As, Cd, Cr, Cu, Hg, Pb and Zn and regional environmental loads plotted. Differences between the median values of the two levels are generally small. However, the concentrations of certain elements like P, Pb and S are significantly greater in the upper layer reflecting contamination from agriculture. In certain regions, the rate of sedimentation was fairly fast such that the environmental effects of ore mining in Transylvania and southern Slovakia as well as those of heavy industry in northern Hungary can be observed in samples from the lower level. The main factor controlling the geochemical pattern in Hungary is the predominance of young (Pleistocene or Miocene) clastic sediments at the surface. Approx. 90% of the surface is covered by these young sediments. This kind of survey has the disadvantage of not providing enough contrast to differentiate geologically dissimilar areas but it has the advantage to provide regional surface background geochemical data and it helps to outline areas of possible surface contamination. Based on the results of this survey we conclude that it would be much better to sample smaller, but geologically homogeneous areas in mountainous terrain to obtain data characteristic of the geochemical background of lithologic units. This approach would mean a sampling density of a few tens of km 2/sample for hilly areas, and a few hundred km 2/sample for lowland areas.

Geochemical and metallogenic provinces: a discussion initiated by results from geochemical mapping across northern Fennoscandia

The concept of metallogenic province has won general acceptance in economic geology. It is agreed that metallogenic provinces offer good opportunities for exploration of new ore deposits. The concept of geochemical province is also established, but its exact meaning is disputable. In this paper, geochemical province is used as an abnormal spatial distribution of an element or element combination in a particular sample type as measured by a particular analytical technique. The practical consequences of this concept of geochemical provinces in mineral exploration seem not to have been fully utilized, although geochemical and metallogenic provinces could only be different manifestations of the same regional features. The location of geochemical provinces should be of greater importance in exploration than the location of metallogenic provinces, simply because a geochemical province can be identified early in an exploration program, while a metallogenic province cannot be defined until a number of ore discoveries has already been made. We have compared metallogenic provinces with geochemical provinces obtained by regional geochemical mapping in Fennoscandia (mainly data from the Nordkalott Project) and other places such as England, Wales, Germany and Alaska in order to study to what degree large-scale geochemical dispersion patterns can be associated with clusters of economically interesting mineral deposits. It is concluded that metallogenic provinces lie within or coincide with a number of geochemical provinces, some of which may have causal relationships with the ore deposits. It can be assumed that an enormous hierarchy of large and small geochemical provinces exist at the earth's surface. The main features of this system would be detectable by worldwide low-density geochemical mapping. Such mapping should be performed in order to shorten the time lapse between possible rises in the demand of certain raw materials and their supply. Worldwide geochemical mapping would also contribute to a better understanding of major geochemical processes of the Earth and provide baselines for environmental research.